This invention relates in general to the fields of medicine, pharmacology and biochemistry, and more particularly to prodrugs capable of delivering a drug across the blood brain barrier including a prodrug made of a fatty acid-dopamine conjugate that is effective as an appetite-suppressant.
There are many obstacles to developing treatments which allow the delivery of a drug to an active site in the body. Ingestion of a drug often is not possible because many drugs will not survive the environment of the stomach. Thus, easy and safe self-administration of many drugs is not available. A drug, of course, may be injected directly into the blood stream of a patient. However, because some drugs do not survive for very long in the bloodstream, frequent injections at great inconvenience to a patient may be necessary. The inability of a drug to survive in the bloodstream may be overcome in certain instances by increasing the dosage or by increasing the frequency of administration. However, increasing the dosage can result in undesirable side effects and increasing the frequency of administration only adds inconvenience.
The delivery of a neuroactive drug to the central nervous system (CNS) via the bloodstream involves an extraordinary obstacle; the drug must be capable of crossing the blood brain barrier. The blood brain barrier may loosely be regarded as a biological exclusion barrier involving both passive and active transport, which barrier controls the exchange of materials between the plasma and the central nervous system. Many drug substances are unable to pass through this barrier in efficacious amounts or at all. Thus, there is a serious need for a mechanism for introducing a drug across the blood brain barrier and into the CNS.
Efforts have been taken to enhance the ability of certain drugs to pass through the blood brain barrier. Investigators have attempted to mask the polar groups of a drug to produce more lipophilic derivatives, as lipophilic compounds are believed to cross the blood brain barrier more readily than hydrophilic compounds. For example, diacetyl and triacetyl esters of dopamine and norepinephrine have been made to mask the hydroxyl groups of these compounds and produce more lipophilic derivatives. This investigator has formed an ester bond between .gamma.-aminobutyric acid, a drug which is unable to cross the blood brain barrier, and a "carrier" molecule having an enhanced ability to cross the blood brain barrier. The carrier-drug conjugate shares with the carrier the ability to cross the blood brain barrier. Once in the CNS, the conjugate itself may be active. However, it is believed that the ester bond between the carrier and drug is broken in the CNS to release the drug in its native form. This may occur due to the general presence of active, non-specific esterases throughout the CNS.
Appetite-suppressant drugs have been sought for many years. Dopamine is believed to be involved in the neuropathways responsible for appetite-suppression. Dopamine itself is not used as an appetite-suppressant because it does not readily cross the blood brain barrier. Drugs which closely resemble the structure of dopamine and which will cross the blood brain barrier have been used with some success as alternatives to dopamine.
The most widely used appetite-suppressant drugs are generally based on derivatives of amphetamine, which structurally resembles dopamine and has some properties which correspond to a dopamine agonist. Unlike dopamine, sufficient uptake of amphetamines across the blood brain barrier does occur to produce a biological effect. However, amphetamines have many serious cardiovascular and neuropsychiatric side effects, as well as a tendency to develop tolerance, the increasing resistance to the usual effect of the drug over time. At worst, tolerance to a drug renders the drug useless. At best, tolerance generally encourages the use of higher drug doses, increasing the possibility of undesirable side effects. In animal tests, tolerance to certain of these amphetamine derivatives has been shown to develop within one day after one dose. Using amphetamine itself, tolerance has been shown to develop within 3-15 days.
Dopamine is also known to play a crucial role in several neurologically related disorders. For example, Parkinsonism is a striatal dopamine deficiency. Because dopamine (and related catacholamines) does not cross the blood brain barrier Parkinsonism is treated with L-Dopa, a precursor to Dopamine. This treatment, however, is at the expense of a wide variety of undesireable side effects, including hallucination. Dopamine agonists which are used in the treatment of hyperprolactinemia associated with pituitary adenomas or amenorrhea also induce undesireable side effects. Thus, there is a serious need for delivering dopamine itself or dopaminergic agents directly to the brain.